The present invention relates generally to apparatus for making bags from a U-folded web of heat-sealable material, and more particularly, to apparatus for controlling the width of plastic bags made on rotary bag making machines.
Rotary bag making machines are well known in the art. Typically, these machines include a rotating drum having a plurality of sealing stations equally spaced about the drum periphery. A web of thermoplastic material, folded in the machine direction to form what is known in the art as a U-folded web, is continuously fed onto the sealing stations at the drum periphery.
Clamp bars, moved into registry with the sealing stations and against the U-folded web, act to hold the web at the sealing stations as the drum rotates. During the course of rotation, a heated wire, carried by the drum at each sealing station, moves against the web. The wire cuts through the two layers of the U-folded web and simultaneously welds the severed layers together to form a seam. This forms a bag wherein the fold in the web defines the bottom of the bag and the side edges of the bag are formed at the welded seams.
A plurality of heated wires are carried by the drum, one wire being associated with each station. Movement of the wires against the web can be controlled by a suitable cam, wherein the drum, in rotating about the cam, causes the wires to move, with respect to the drum surface and at the appropriate intervals into and out of engagement with the web.
The machine, as described thus far, is conventional and produces a bag which is as deep as the folded web and as wide as the peripheral distance between adjacent sealing stations.
With prior art apparatus, the depth of the bag is changed by slitting the web to an appropriate width prior to folding it. However, changing the bag width is not possible except by welding and severing at every second or third sealing station, for example, to double or triple the width of the bag.
Bag making apparatus as typified, for example, by U.S. Pat. No. 4,331,502 and 4,115,183, among others, utilize a tucking mechanism for increasing the rate of production. In this respect, the sealing stations are closely spaced about the periphery of the rotating drum and a tucking mechanism operates to tuck the web inwardly from the periphery of the drum. In this apparatus, the depth of the tuck, determines the width of the bag. The means for controlling the depth of the tuck, as described in these patents, is a fixed cam track which is arranged so that rotation of the drum with respect to the track causes roller members to move inwardly from the periphery of the drum.
One disadvantage of the '502 and '183 bag making machines is the uncertainty in predicting an exact dimension between the welded seams forming the side edges of the bag. In this respect, the thermoplastic web shrinks or "pulls back" from the heated edge. The amount of this shinkage depends upon a number of variables such as the heat generated by the hot wire, the thickness of the thermoplastic web and the composition of the web, among others. Accordingly, when designing a bag making machine for a particular width of bag, these factors are all taken into consideration. However, the final dimensions of the bag are not likely to be known until the machine is actually put into operation. Even then, the width of the bag may vary as conditions change during the course of the run, or as thermoplastics of slightly different compositions are used. Accordingly, a machine designed for a specific size of bag may be found to produce either oversize or undersize bags when the machine is actually put into production.
Even though the bag actually produced by the machine may be oversized or undersized by as little as fractions of an inch, either situation is undesirable. In this respect, it may be deceptive to advertise a bag which is under a desired size, as being the desired "full" size; as for example, to say a bag which measures only 6 and 7/8 inches as being a seven inch bag.
On the other hand, if the bag is over sized, material is being wasted in the manufacture of the bag. For example, it is not uncommon to manufacture bags at the rate of hundreds per minute. At these rates, even fractions of an inch per bag translates to losses in excess of hundreds of feet of material per hour.
In prior art machines having just a rotating drum and with no tucking mechanism, there is no practical method of correcting for undersized or oversized bags. With machines of the prior art, using a tucking apparatus, the only practical manner of correcting bag size is to recut that portion of the cam controlling the tucking apparatus. This procedure is not only costly, but results in expensive downtime of the machine while the cam is being recut.
Another drawback of machines of the prior art is the inability to manufacture bags in sizes other than multiples of the size for which the machine was manufactured. For example, with prior art machines one could manufacture bags which are double or triple width simply by sealing and severing the bag at every second or third sealing station. However, it is difficult, if not impossible, to manufacture one size of bag, and then adjust the machines for manufacturing another bag which is not a multiple width of the first bag.
The bag making machine of the present invention is of the type utilizing a tucking mechanism for tucking the thermoplastic web radially with respect to the periphery of the drum. Prior art apparatus, as typified by the patents mentioned above, and which use a fixed cam to operate the tucker mechanism for tucking the web inwardly from the periphery of the drum, have certain limitations. For example, space restrictions towards the center of the drum severely limit both the number of tucking stations for a given size drum and the maximum depth of tuck. Also, application of the web onto the drum is difficult with inward tucking because the tucking members must be moved aside to permit the web to have access to the periphery of the drum.
In the present invention, the drawbacks of the prior art tucking apparatus are obviated by arranging the tucking members, and the cam, to tuck the web outwardly from the periphery of the drum. With this outward tucking arrangement the spacing limitations decrease as the tuck length increases. In addition, outward tucking readily accommodates an increase or decrease in the depth of tuck without creating mechanical interference during application of the web to the sealing stations on the periphery of the drum, and it accommodates the application of the web to the drum without interference from the tucking members. Also, with outward tucking, the tucking members can be retracted from the position of full tuck in order to relieve tension on the web just prior to severing and sealing the web. As a further feature, the cam can be made adjustable so as to facilitate changing, or correcting, the tucked depth and therefore the width of the bag.